Fluid mixture separation



Sept. 13, 1960 R. E. sATTLER FLUID MIxTuRE SEPARATION Filed April 2, 1956 mmm INVENTOR. R. E.SATTLER /L/wamq' A WOR/vers United States Patent-(f)V FLUID MIXTUREv SEPARATION Robert E. Sattler, Bartlesville, Okla.,` assigner to Phillips Petroleum Company, a corporation of Delaware Filed Apr. z, 1956, ser. No. 515,644 5 claims. (ci. 62`z1 volves initially removing constituents heavier than pro-l pylene in an absorption column. The resulting gaseous efuent is then passed through three fractionation columns operated as a demethanizer, a deethanizerand an ethylene fractionator, respectively. The feed to the final ethylene fractionator is essentially a binary mixture of ethylene and ethane. However, small amounts of methane may be present. If the feed were solely a binary mixture, the

reflux rate in response to av measurement of temperature in the upper region of the column. However, the presence of methane in the system complicates the control procedure. As long as the methane concentration remains constant, the temperature control is adequate. If

the methane concentration inthe feed stream should increase, however, the concentration of methane in the upper region of the column increases because the methane is removed with the overhead product. An increase in concentration of methane in the upper region of the column results in alower temperature at thisV region. This lower temperature actuates the reflux control to decrease the rate of reflux.` The increased methane content, in itself, decreases the purity of the overhead ethylene product, and the decreased reflux rate further decreases the overhead ethylene purity. The temperature control is thus not effective. Y

A control system is provided in accordance with the present invention Which overcomes the difficulty previously mentioned. This control system is based upon an analysis of a sample in the upper region of the column to determine the ethane concentration. In response lto t-his analysis, the temperature controller is reset to adjust the reilux rate. This serves to override the temperature' control so that the ethane content in the upper region of the column remains within predetermined limits at Accordingly, it is an object of this invention to provide anv improved method of and apparatus for controlling the separation of fluid mixtures by fractionation.

` Another object is to provide a system for separating a first constituent from a second constituent of a iluid mixture in the presence of small amounts of a third con-Y stituent having a lower boiling point than either of the first two constituents;

`Other objects, advantages and featureslof the invention should become apparent from the following detailed description which is taken in conjunctionwith the accom;- panying drawingl inwhich:

Figure l is a schematic representation of aifractionanorl having the control system of this invention incorporated therein; and,

Figure 2 is a schematic representation of an mfrared analyzer which can be employed in the control system ofy Figure l.

Referring now to the drawing in detail, and to Figure 1,'.

f in particular, there is a fractionation column 10, which 12 which adjusts a valve 13. A kettle product stream is;

is supplied with a feed mixture to be separated by means of an inlet conduit 11. The ilow through conduit 11is maintained at a predetermined rate by flow controller removed from the bottom of column 10 through an outlet conduit 14 which has a control valve 15 therein. Valve! 15 is adjusted by a liquid level controller 16 which sensesV the liquid level in the bottom of column 10. A portion.- of the kettle product is directed by a conduit 17 throughl a reboiler 18 and returned to a lower region of column,`

10. A heating medium is directed through reboiler 18 by means of a conduit 19 which has a valve 20 therein-f that is adjusted by a oW controller 21.

, Gases are removed from the top of column 10 througha conduit 23 which communicates with a separator 24: A predetermined pressure is maintained in column 1 0.

by a pressure controller 25 which regulates a valve 26 in conduit 23. Any condensed materials are removed fromv separator 24 through adrain conduit 27. Gases are` L removed from separator 24 through a conduit 28 which;

communicates with the inlet of a compressor'29. An` ethylene product outlet conduit 30 communicates with the outlet of compressor 29. A conduit 31 communicates;`

. f. between the outlet of compressor 29 and a surge tank 32.; column could be controlled effectively by adjusting thev A condenser 33 is positioned in conduit 31. `A conduit 35 extends from the outlet of tank 32 through aco'olerv 36 to the upper region of column 10. 'Ihe rate of ow of reliux through conduit 35 is adjusted by a controlV y valve 34 which is actuated by a temperature controllerI 37 in response to a measurement of temperature in th upper region of column 10. l

`A sample line 39 communicates between an upper` region ofv column 10 andthe inlet of an analyzer 401: The sample stream is vented from analyzer 40 through a conduit 41. Analyzer 40 is `adapted to detect the con` centration of ethane in the sample stream and to provide an output signal representative of this concentration.'` This output signal is applied through a controller 66 to reset temperature controller 37 so as to control `valve 36 in such a manner as to maintain the measured ethane concentration within predetermined limits.

In order to explain more fully the operation of the control system of this invention, reference will be made to a particular separation of a feed stream containing primarily ethane and ethylene. This feed stream, which` is introduced into column 10 through conduit 11, normallycomprises primarilyethylene and ethane in a mole ratio., of about 41/2 to l.` Small amounts of methane, acetylene and propylene normally are present. je

Column 410 is operated at an overhead pressure of V1'35v p.s.i.a., and a bottom pressure of 140 p.s.i.a. The over.,l head temperature is -65 F. and the bottom temperature is 28 F. The gases removed from compressor29 are at a pressure of 310 p.s.i.a. and a temperature of 65 F.v These gases are cooled by condenser 33 to a tempera-1 ture of 18 F. The liquid removed from tank 32is further cooled to a temperature of -55 F. before being returned to column 10 as reflux. Analyzer 40 is set to' operate to increase the opening of valve 34 whenever the measured concentration of ethane increases above a set ethane concentration. Analyzer 40 detects this ethaneV increase. Valve 34 is then opened suiciently to maintain Patented Sept. 13, 1960" the measured vconcentration less than the set'value by increasing the reflux rate. Obviously, the indicated operating pressures and temperatures are merely representative of one po'ssiblemode of operation. These conditions depend upon the desired separation and the composition of the feed. Y

' In Figure 2 there is illustrated an analyzer which can be employed to advantage as analyzer 40 inthe control system of, Figure l. Theanalyzer of FigureZ comprises a sample cell 50 through which the sample stream removed from column is circulated. A first beam of iufraredradiation from a source S1 is directed by a reector 52 throught cell 50 and a second cell 53 to impinge upon a rst temperature sensitive resistance element 54. A second beam of radiation fromy source 51 is directed by a rellector 55 through cell 50 and a third cell 56 to impinge upon `a second temperature sensitive resistance element 57. The analyzer is sensitized to measure thek concentration `of ethane in sample cell 50` This is accomplished by lilling cell 53 with a pure` sample ofr ethane. `Cell 56 is lled with a non-absorbing gas, such as nitrogen. This cell functions merely to equalize the optical paths. It should be evident that lal1-increase in concentration of ethane in sample cell 50 does not decrease the radiation irnpinging upon element 54 because cell 53 absorbs the wave lengths of radiationwhicn-are absorbed by the ethane in cell 50. However, an increase in ethane concentration in cell 50increases'thegamount ofV radiation absorbed by the beam directed toward element 57. This changes the resistance of element `57i by` changing the temperature thereof.

First terminals of elements 54 and 57 are connectedr 4 purpose. A mass spectrometer, for example, is another instrument which can be so used.

It should be evident that the principles of this invention are not restricted to the particular ethane-ethylene separation herein described. The invention can be employed to advantage whenever it is'desired to separate tirst and second constituents of a uid mixture which may also contain small amounts of `a third constituent having a and second constituents.

While the invention has been described in conjunction with a present preferred embodiment, it should be eviboiling point lower than the boiling point of the iirst dent that it is notlimited thereto.

. What is claimed is;

1. The method of separatinga uid mixture including first, second and third components having progressively lower boiling points in the order named which comprises passing a stream of such a mixture into a fractionation Zone, withdrawing an overhead stream from said zone, condensing at least a portion of said 'overhead stream and returning same to said zone as rellux, withdrawing ra bottoms stream fromr said zone, measuring the tempenature in `an upper region of said zone, regulating, the

rate offlow of reflux to said zone in response to the temperature measurement to tend to maintain the measuredtemperature constant, withdrawinga sample stream y from an upperV region 'of said zone, measuring the to one another and to the iirst terminal of a voltage source 58. The second end terminals of elements 54kk and 57 are connected `to the respective end terminals of potentiometer 60. The contactor ofpotentiometer 6l! is connected to the second terminal of voltage source S8.

nectedto a reversible servo vmotor` 62.` Thecircuit is adjusted so that motor 62 rotates in afirst direction if the resistance of element 57 increases andy rotates in a second direction'y if theresistance of element 57 decreases. The drive shaft of motor 62 is mechanically coupled to the contacter of potentiometer 60 to move this contacter y The second terminals of elements' 54 and'57` vare also connected to the respective input terminals vofan amplilier 61.` The output terminals of amplifier 61 are conin a direction to restore the bridge circuit toa balanced condition. The movement of the contacter of potentiomn eter 60 to restore this balance is indicative of the change in resistanceof element 57. This,-in turn, isa function of the change in ethane concentration in sample cell 50.: w

, The drive shaftof motor 62 is also connected to the yThis regulated output air vpressure can'be transmitted to temperature controller 37 of Figure l' to resety the con-y trol pointy of. this controller. This, in turn, adjusts the setting of control valve'34. Controller 37 can be a conventional air-operated controller with reset such as is described in Catalog No. 8904 (1943) of said Brown Instrument Company, for example. i

It should be evident that the control system of this invention is not restricted to any particular ethane analyzer. The illustrated infrared analyzer can be employed to advantage to measure the concentration of ethane, but other known analyzers can also be employed -.QI thisA contactor of a'telemeteringpotentiometer `64. A voltage lamount ofr said iirstcomponent `insaid sample stream, y i y andfurther controlling the rate of flow of refluxto said zone responsive to the last-mentioned measurement to tend tomaintainthe concentration of said first kcomponent rless than a predetermined value.y

2. The method of separating a fluidmixture comprising ethane' and ethylene and which may contain smalll amounts yof methane which comprisespassing a stream `of such a `mixture into a fractionation zone, withdrawing an overhead stream fromsaid zone, condensing at least f ra portion of saidjoverhead stream kand returning same to said zoner as rellux, withdrawing a bottoms streamv from said zone,r measuring the temperature inr an uppery region ot said zone, regulating the rate of flow ofreflux to said zone` in response to the temperature measurement to tend to maintain the measured temperature constant, withdrawing-a sample Ystream from an upper region of said zone, measuring the concentration of ethane in `saidfsample stream,r and further controlling the rate of yflow lof redux to said `zone responsiveto the measure- `ment to maintain the measured concentration of, ethane 'less thana predetermined value.

. 3.- In "a 'system adapted to separate a duid mixture including grst, y second andfthird components having .progressively lower boiling points in the order named comprising'a fractionation column, means to introduce such a liuid 'mixture intosaid column, means to withdrawan overhead stream from said column, means to condense at least a portion of the withdrawn overhead stream andto return same to said column as reflux, and means to withdraw a bottoms stream from said column; control apparatus comprising means to measure the temperature in an upper region of said column, means ref sponsive to said means to measure 'temperature to control the rate of flow of reflux to said column, means yto withdraw a sample 'stream from an upper region of said f column, means to analyze said sample stream to measure the concentration of said iirst component, and means responsive to said Vmeans to analyze to override said means tokcontroll lto maintain the concentration of said first component '-in said `sample stream less than a predetermined value.

4. Apparatus to separate a fluid mixture including rst,

second and third components having progressively lower n boiling points inthe rorder vnamed comprising a fractionation column, means 'to introduce such a fluid .mixture into said column at a predetermined rate,` means to withdraw a gaseous overhead stream from said column at such a rate as to maintain a constant pressure in said column, means to `supply heat to said column at a predetermined rate, means to withdraw a bottoms stream from said column at a rate such as to maintain a constant liquid level in `said column, means to condense at least a portion of said gaseous overhead stream and to return same to said column as reflux, means to measure the temperature in an upper region of said column, means responsive to said means to measure temperature to control the rate of ow of reux to said column, means to withdraw a sample stream from an upper region of said column, means to analyze said sample stream to measure the concentration of said rst component, and means responsive to said means to analyze to override said means to control to maintain the concentration of said :first component in said sample stream less than a predetermined value.

5. Apparatus to separate a fluid mixture including first, second and third components having progressively lower boiling points in the order named comprising a fractionation column, means to introduce such a uid mixture into said column at a predetermined rate, means to supply heat to said column at a predetermined rate, means to withdraw a bottoms stream from said column at a rate such as to maintain a constant liquid level in said column, means to Withdraw a gaseous overhead stream from said column at such a rate as to maint-ain a constant pressure in said column, means to compress said overhead stream, means to cool Ia portion of the compressed stream, means to return the cooled stream to said column as reflux, means to measure the temperature in an upper region of said column, means responsive to said means to measure temperature -to control the rate of ow of reux to said column, means to withdraw a sample stream from an upper region of said column, means `to analyze said sample stream to measure the concentration of said rst component, and means responsive to said means to analyzeto override said means to control to maintain the concentration of said irst component in said sample stream less than a predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 2,156,234 Bays Apr. 25, 1939 2,336,097 Hutchinson Dec. 7, 1943 2,475,957 Gilmore July 12, 1949 2,553,469 Pellettere May 15, 1951 2,599,133 Schilling June 3, 1952 2,673,297 Miller Mar, 23, 1954 2,697,789 Skarstrom Dec. 21, 1954 2,703,844 Miller Mar. 8, 1955 2,721,942 Friel Oct. 25, 1955 2,762,208 Dennis Sept. 11, 1956 2,764,536 Hutchins Sept. 25, 1956 2,835,116 Miller May 20, 1958 2,885,863 Berger May 12, 1959 FOREIGN PATENTS 164,584 Australia Aug. 15, 1955 

